1. Field of the Invention
The present invention relates to the construction of lead frames for semiconductor devices.
2. Background Art
Copper lead frames are employed extensively in semiconductor device packages primarily due to their high thermal conductivity. Often, the lead frame is preplated with a suitable coating metal prior to encapsulation because the coating significantly reduces the number of assembly steps typically required for post-assembly coating processes. The preplate coating protects the exposed lead frame, and promotes wire bonding and solderability of the leads after encapsulation.
Optimal lead frame coatings should be free of lead (Pb), corrosion resistant, oxidation resistant, resistant to high temperatures (over 200.degree. C.), wire-bondable, and solderable after assembly and burn-in processes. Additionally, the coating should be cost effective and cosmetically acceptable.
The prior art discloses lead frame coatings in which the outermost coating layers comprise nickel and palladium. However, lead frames are required to possess a relatively high solderability coverage as measured by standard solderability tests. In some cases, it has been found that such preplated leads are not as solderable as might be desired.
Thus, there is a need for a lead frame structure which is readily solderable, while still exhibiting the desirable characteristics described above.